The use of moisture-curable one-component and two-component isocyanate-based coating compositions in the production of coatings for substrates is known in the art. As an illustration of a one-component system, U.S. Pat. No. 4,456,658 discloses a process for coating polyvinyl chloride sheet products with a moisture-curable, clear coating comprising a binder which consists essentially of at least one polyisocyanate which is liquid at room temperature. The polyisocyanate binder is further characterized at column 2, lines 12-17 of the '658 patent as being in particular polyisocyanates containing biuret or isocyanurate groups and optionally uretidione groups and having an average isocyanate functionality above 2, preferably from about 2.5 to 6. Unfortunately, the '658 patent is limited to coating polyvinyl chloride sheet products, which represents a narrow market niche as compared to the wide variety of substrates in the marketplace that could benefit from a suitable one-component coating. The coating compositions of the examples of the '658 patent also have the disadvantage of slow drying times when cured with atmospheric moisture at room temperature.
As an additional illustration, U.S. Pat. No. 4,292,350 discloses a process of coating substrates comprising applying to the substrate a moisture-curable, solvent-free or low solvent lacquer composition containing as a binder an isocyanate mixture. The isocyanate mixture described in the '350 patent is a mixture of (a) an organic polyisocyanate having an average isocyanate functionality of greater than 2 which contains biuret, urethane and/or isocyanurate groups and has an isocyanate content of from about 13 to 30% by weight, and (b) at least one monoisocyanate having a defined structure. The monoisocyanate is further described at column 3, lines 18-22 of the '350 patent as being suitably prepared by reacting excess quantities of a diisocyanate with a hydrocarbon-containing hydroxyl compound, followed by removal of unreacted excess diisocyanate by distillation using, for example, a thin layer evaporator. The process of this patent is more cumbersome than might be desired, particularly in regard to the need for the isocyanate mixture and the need for removal of excess diisocyanate from the prepared monoisocyanate component.
In the production of heat-cured coatings, conventional methods for the heat-curing of monomeric diisocyanates have frequently been problematic in view of the toxicity concerns typically associated with employee exposure by the end user to volatile monomeric diisocyanates.
U.S. Pat. No. 5,144,031 discloses both heat-crosslinkable, one-component blocked-isocyanate compositions and two-component coating compositions which are said to harden at room temperature or at a slightly elevated temperature. The coating compositions of the '031 patent contain isocyanurate group-containing polyisocyanates produced by the steps of: (a) partially trimerizing the isocyanate groups of a cycloaliphatic diisocyanate in the presence of a trimerization catalyst and optionally in the presence of an inert solvent, (b) terminating the trimerization reaction at the desired degree of trimerization, (c) removing unreacted starting diisocyanate and any other volatile constituents, and (d) prior to step (c), adding 1 to 30 % by weight of a monohydric alcohol being an ester alcohol, based upon the weight of the diisocyanate. Unfortunately, this process is more complicated than might be desired, particularly with the requirement for the step (c) removal of unreacted starting diisocyanate after addition of the monohydric alcohol. In addition, the presence of the ester alcohol necessitates that a urethanization reaction takes place at the site of the ester alcohol on the prepolymer, in addition to the trimerization reaction that takes place with respect to the isocyanate moieties, rendering a more complex reaction sequence than might be desired.
Thus, a simpler process for coating a wide variety of substrates, not limited to the polyvinyl chloride substrate of the '658 patent and not employing the isocyanate mixture of the '350 patent, utilizing a heat-curable coating composition having a fast drying time would be highly desired by the one-component and two-component urethane coatings community. The present invention provides such a simplified process. The present invention also provides a solution to the toxicity problem that can be associated with monomeric diisocyanates by employing a low toxicity coating composition that is also low in VOC.